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20                                              INTRODUCTION.
said as to how and where the test shall be taken. This omission is covered by a general understanding in the trade so that there is seldom any trouble in the case of standard structural shapes. Where-ever it is possible the test piece is taken so as to leave two parallel rolled surfaces on the test bar, the other two sides being machined. This can readily be done with plates, beams, channels, angles and similar shapes. In small rounds the whole piece is taken as it comes from the rolls. In the case of plates it is understood that the test piece is to be taken lengthwise of the plate unless stated otherwise in the specifications. In f orgings, however, no absolute standard can be given, but it is usual to cut a test from a prolongation of the piece at a short distance below the surface. In many cases this is unnecessary, and it will suffice to forge a small bar from the heat and finish this either at a small hammer or at a rolling-mill. In other cases, like armor plate and cannon, stringent provisions are incorporated in the specifications.
The results obtained from test pieces of different shape are not the same. The general section, whether round or rectangular, makes a difference, and in a rectangular piece the relation of the width to the thickness influences the result. It will be seen that this latter fact is important in cutting strips from angles or flats of varying thickness. Needless to say that the length is the one predominant factor. Just before breaking there is a drawing out of the bar in the immediate neighborhood of the place where it is going to break, and this local stretch will be a greater proportion of the total in the case of a bar two inches long than with a bar ton inches long. In order that records shall be comparative, the length of eight inches is used throughout England and America; except for forgings and castings, in which cases a 2-inch test is often used, as it is both- inconvenient and expensive to get the longer piece. In foreign countries the standard length is 200 millimeters =7.87 inches, so that the results are fairly comparable with our 8-inch test.            
The general laws may be thus summarized, the data from which the conclusions are drawn being given in Chapter XVI.
(1)   A rolled round will give the best results if tested in the shape in which it leaves the rolls.   If the outside surface is removed by machining the elongation will be reduced.
(2)   The tensile  strength of  a plate as  determined by the grooved  (marine)   section will be from 6500 pounds to 12,500